JP3258038B2 - Optical communication system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system applicable to, for example, a subscriber system.

[0002]

2. Description of the Related Art As an example of a conventional optical communication system of an optical subscriber system, a system as shown in FIG. In this figure, reference numeral 1 denotes an optical transmitter of an intensity modulation system. The wavelength of this optical signal is transmitted at λ ₁ and transmitted to the star coupler 3 via the optical fiber 2. The signal distributed by the star coupler 3 is connected to optical receivers 1 (7), (8), (9) of the direct detection system of each subscriber by optical fibers 4, 5, and 6. Such a transmission system is generally considered as a television signal distribution / transmission system.

[0003] By the way, to a subscriber who desires a second service different from the first service, an optical transmitter 2 (1
Consider the case where a service is provided using (0). In such a case, in the related art, there is a method of performing wavelength multiplex transmission,
This is shown in FIG. The wavelength of the optical receiver 2 by the intensity modulation is λ
2 and are combined through an optical multiplexer (11) to form an optical fiber (2),
Transmission is performed using (4) and (6). Then, an optical demultiplexer (14) and an optical receiver 2 (15) are arranged for a subscriber who desires the second service, and the second service is received by the optical receiver 2 (15). In this case, it is necessary to insert a wavelength filter or a demultiplexer (12) or (13) also in the optical receiver of the subscriber who only needs the first service. As another method, a method is conceivable in which the second service is provided using a new optical fiber (17) as shown in FIG. In this system, when each subscriber desires the second service one after another, the subscriber switches to a star optical fiber network.

[0004]

In the conventional multiplexing system, whether the subscriber who does not receive the second service is required to improve the device or whether a new optical fiber transmission system is installed. is there. The former causes a great deal of trouble for subscribers who do not receive the second service, and the latter is extremely costly because conventional equipment cannot be used at all.

An object of the present invention is to solve the above-mentioned problems and to provide an optical communication system which can use a conventional transmission line without changing a device of a subscriber who does not receive the second service. And

[0006]

According to a first aspect of the present invention, a first transmitter for intensity-modulating and outputting a digital signal and a first receiver for directly detecting the intensity-modulated digital signal are used. Intensity modulation at the first wavelength, a first transmission system that performs digital signal optical communication by direct detection, and a constant light intensity at the second wavelength using a second transmitter and a second receiver. A second transmission system that performs optical communication of a phase modulation system or a frequency modulation system is multiplexed, and an optical communication system that performs information transmission by using the multiplexed signal, wherein the first receiver,
Receiving the multiplex signal as it is, extracting the output signal of the first transmitter from the optical intensity modulated signal of the multiplex signal, the second receiver receives the multiplex signal as it is, the second reception An optical communication system characterized in that the transmitter extracts the output signal of the second transmitter by utilizing the fact that the transmitter has higher optical frequency selectivity than the first receiver.
In a second aspect, the second receiver combines a local light oscillation element that oscillates light of a reference wavelength, light that remains as the multiplexed signal for information transmission, and light from the local light oscillation element. 2
A 2 × 2 combiner, and a plurality of photoelectric conversion elements for receiving the light combined by the 2 × 2 combiner and converting the light into an electric signal, and extracting a difference signal between the plurality of photoelectric conversion elements. This is an optical communication system according to the first invention. Third
In the invention, the second receiver is constituted by a heterodyne receiver, and the second transmission system sets an intermediate frequency of the second transmission system in a band equal to or more than a base band band of the-transmission system. An optical communication system according to the first or second invention, wherein the optical communication system is set. According to a fourth aspect, a first transmitter (101) for intensity-modulating a first digital signal and outputting the first digital signal and a first receiver (125) for directly detecting the intensity-modulated first digital signal are provided. A second digital signal is intensity-modulated and output to a first transmission system performing optical fiber communication of a first digital signal by intensity modulation and direct detection at a first wavelength using Intensity modulation / direct detection at a second wavelength different from the first wavelength using a transmitter (103) and a second receiver (126) for directly detecting the intensity-modulated second digital signal. First by method
The second transmission system for performing the communication of the digital signal using the optical fiber is an optical communication system multiplexed using optical multiplexers / demultiplexers (105 and 122).
And a third receiver (128) using a third transmission system for performing phase modulation or frequency modulation communication with a constant light intensity using a third wavelength using the optical fiber. A multiplexed signal obtained by multiplexing the output signal of the first transmitter, the output signal of the second transmitter, and the output signal of the third transmitter passes through the optical fiber, and An output signal of the first transmitter is separated from the multiplexed signal by the transmitter, and the first receiver receives the output signal of the separated first transmitter;
The light intensity modulated signal of the multiplexed signal after the separation is obtained by adding the light intensity modulated signal of the output signal from the second transmitter to the signal of constant light intensity which is the output signal from the third transmitter. Wherein the second receiver receives the demultiplexed multiplexed signal as it is, extracts the output signal of the second transmitter from the optical intensity modulated signal of the demultiplexed multiplexed signal, Receive the multiplexed signal as it is after the separation, and utilize the fact that the third receiver has a higher optical frequency selectivity than the second receiver.
The optical communication system is characterized in that an output signal of the transmitter is extracted. According to a fifth aspect of the present invention, there is provided one
An optical communication system according to the first or fourth aspect of the present invention, wherein star couplers of N pairs are arranged.

[0007]

In the optical communication system having the above configuration, the first optical fiber communication of the intensity modulation direct detection system is performed at the first wavelength.
Uses the principle that even if transmission by phase modulation or frequency modulation using the second wavelength is multiplexed into the transmission system, no large noise is added to the first transmission system. That is, since the second transmission system performs phase modulation or frequency modulation, the light intensity is always constant. Even if such an optical signal is added to the first transmission system, only a slight increase in shot noise is generated, and no large noise is added to the first transmission system. Further, since the optical heterodyne or optical homodyne method is generally used to demodulate the phase modulation or the frequency modulation, the second transmission system has high sensitivity. Therefore, the second
May be smaller than the optical signal intensity of the first transmission system, and it can be said that the increase in shot noise of the first transmission system is extremely small.

In the second transmission system, the optical frequency selectivity is large due to optical heterodyne or optical homodyne, and the optical signal of the first transmission system does not cause a problem as noise. The present invention has been made based on the above principle.

[0009]

An embodiment of the present invention will be described with reference to the drawings.

First, the basic configuration of the present invention is shown in FIG. Intensity modulation by the optical transmitter 1 (1) sends out the optical signal of the wavelength lambda _1, the optical fiber 2,1-N star coupler 3 through the optical fiber 4, 5, 6, the direct detection method for each subscriber The optical receivers 7, 8, and 9 are connected. The first service is provided in this system, and the second service is added. The second service is provided by the optical transmitter 2 (20). The optical transmitter 2 (20) transmits the light of the wavelength lambda _2, performs transmission by frequency modulation or phase modulation. The optical signal transmitted from the optical transmitter 2 (20) is combined with the light from the optical transmitter 1 (1) by the optical distributor (21). On the receiving side, the optical signal is distributed by the optical distributor (22), and the optical signal is added to the optical receiver 2 (25).
The optical receiver 2 is capable of demodulating the optical signal of the optical transmitter 2 and is a receiver using an optical heterodyne or optical homodyne method.

In such a system, the receivers 1 (7), (8),
FIG. 2 shows the waveform of the optical signal received by (9). Although a digital signal is used in this example, an analog signal can be similarly applied. The optical signal is represented by the sum of the signal component of the optical transmitter 1 and the signal component of the optical transmitter 2. Then, the optical receiver 1 receives a waveform whose extinction ratio has deteriorated because the optical transmitter 2 is not intensity-modulated. However, optical heterodyne / homodyne receivers generally have high sensitivity, so that the received light power to the optical receiver may be small. Therefore, the extinction ratio deterioration is relatively small, and the influence on the optical receiver 1 is small. Thus, the optical receiver 1 of the subscriber who does not desire the second service
(7) and (8) can receive the first service without taking any action. In addition, the conventional optical transmission can be used as it is. The optical receiver 2 (25) has high optical frequency selectivity due to local gain and balanced reception, and can receive the signal of the optical receiver 2 without being affected by the optical receiver 1.

FIG. 3 shows an example of the configuration of the balanced receiver. FIG. 1 shows a configuration of a receiver when optical communication is performed by a heterodyne / homodyne method. The transmitted light is incident from an optical fiber 50 and is combined with light oscillated from a local light emitting element (for example, a photodiode) by a light combiner 51 such as an optical coupler. The combined light is distributed to two photodiodes 52 and 53 connected in series in the same polarity direction. The electric signal photoelectrically converted by each photodiode is extracted from the serially connected portion of the two photodiodes, but the intensity-modulated signal component of the transmission signal is canceled. As a result, the signal component transmitted by the first wavelength is not detected, and the signal transmitted by the second wavelength causes interference with light from the local light emitting element, and obtains a signal with a large local gain. be able to.

FIG. 4 shows an example in which the FSK demodulation circuit is realized by an optical circuit in a case where the receiver is not a heterodyne / homodyne receiver. The transmission signal enters from the optical fiber 60.
Optical couplers 61 and 62 for performing an interferometer and optical fibers 63 and
A Mach-Zehnder interferometer is composed of
M conversion is performed. The transmission signal causes interference of the transmission signal due to the photosynthesis, and can cancel the intensity modulation component in the signal of the photodiode serial connection in FIG.

Further, in the heterodyne receiver, the wavelengths used in the first transmission system and the second transmission system are filtered by a filter.
It is possible to separate the signals. FIG. 5 shows the frequency distribution of the output of the amplifier 54 as the IF signal component of the heterodyne receiver shown in FIG. IF signal 71 by optical transmitter
May be set higher than the intensity modulated signal 70 leaked by the transmission system 1. By extracting the IF signal by the bandpass filter 56 having the characteristic 72 for extracting the frequency band of the IF signal, it is possible to suppress the intensity modulation component having a lower frequency than this.

The present invention is applicable to a wide variety of optical networks. Fig. 6 shows TCM (time axis compression single core bidirectional multiplex)
1 is an example of a star-like subscriber system using WDM and wavelength division multiplexing (WDM). (100) is a station, and (111), (117) and (127) are subscribers. In the station, there are an optical transmitter 1 (101) and an optical receiver (102) of the intensity modulation / direct detection system for TCM, and an intensity modulated optical transmitter 3 (103) for providing a different service. There is. These are multiplexed into a single optical fiber (106) by an optical distributor (104) and an optical multiplexer / demultiplexer (105). The optical transmitter 1 (101) and the optical transmitter 3 (10
The wavelengths transmitted by 3) differ greatly, and WDM is realized by the multiplexer / demultiplexer (105). 1 to N star coupler (10
The optical signal is distributed by 7) and the optical fibers (108), (109), (110)
Distributed to each subscriber. Each subscriber has an intensity-modulated optical transmitter 1 (114), (118), (124) and a direct detection optical receiver (11).
5), (119), (125) and optical distributors (113), (120), (123),
An optical multiplexer / demultiplexer (112), (121), (122) is provided to perform TCM transmission. Further, the subscribers (114) and (127) receive the optical receiver 3 (116),
(126) receives the service of the optical transmitter 3 (103) in the station. Using the present invention for this network, the third and fourth
The case where the service is performed will be described below.

FIG. 7 shows an example in which a third service is added. The optical transmitter 2 (140) is a frequency-modulated or phase-modulated optical transmitter, and the transmission wavelength is near the optical transmitter 3 (103). Therefore, they are combined by the optical distributor (130) and distributed to the subscriber via the optical multiplexer / demultiplexer (105). Subscribers (12
7) installs an optical distributor (129) and an optical receiver 2 (128) to receive the third service. Other subscribers can receive the conventional service without changing the device.

FIG. 8 shows an example in which a fourth service is added. In this example, a large amount of information is transmitted from a subscriber to a station. The subscriber (127) adds a frequency-modulated or phase-modulated optical transmitter 4 (150) and a one-to-three optical distributor (151). The wavelength transmitted by the optical transmitter 4 (150) is close to the wavelength transmitted by the optical transmitter 2 (140). Each wavelength arrangement is illustrated in FIG. λ ₁ is the wavelength transmitted by the optical transmitter 1, and similarly, λ ₂ , λ ₃ , and λ ₄ are the wavelengths transmitted by the optical transmitters ₂ , ₃ , and ₄ , respectively. In the figure, the broken lines indicate the transmission characteristics of the optical multiplexer / demultiplexers (105), (122), (121), and (112), and λ ₂ , λ ₃ , and λ ₄ are reflected and output from the same port. It can be seen that the wavelength is The optical signal transmitted from the optical transmitter 4 can be received by the optical receiver 4 (153) in the station. It is a feature of the present invention that even if the number of services increases, a subscriber who does not desire the services does not need to make any change. The insertion loss of the optical dividers (151) and (152) increases as the number of distributions increases. However, since the optical heterodyne / homodyne receiving system has high sensitivity, the distribution ratio is not equalized, and intensity modulation and direct detection are not performed. What is necessary is just to allow large optical power to be distributed to the optical transceivers (103) and (126). Further, the transmission system of the optical transceiver 1 or 3 may be used for controlling collision prevention of the transmission system of the optical transceivers 2 and 4.

FIG. 10 shows an example in which the present invention is applied to an N-to-N star network. Each station (201), (202), (203) is connected to an N-to-N star coupler (204) by an optical fiber.
Each station includes an intensity modulation transmitter 1 (205), (211), (217) and an optical receiver 1 (207), (213), (219) for receiving this signal. Further, a frequency-modulated or phase-modulated optical transmitter 2 (2
06), (212), (218) and the optical receiver 2 (208), (2
14) and (220). There are also optical distributors (209), (210), (215), (216), (222), and (221) for combining or distributing optical signals. Although there are two transmission systems in this network, it is assumed that basic communication is performed by the optical transmitter 1 and the optical receiver 1. That is, communication protocols such as CSMA / CD and implicit token are supported by the optical transmitter 1 and the optical receiver 1, and the optical transmitter 2 and the optical receiver 2 support additional large-capacity transmission.
Also, the transmission wavelength of the optical transmitter 2 is changed for each station,
If the optical receiver 2 performs selective reception, the network can have a very large capacity. In such a case, the optical transmitter 1 and the optical receiver 2 may be used to control communication between the optical transmitter 2 and the optical receiver 2. In this case, the procedure may be determined, such as making a reservation with the partner station using the optical transmitter 1 before starting the transmission with the optical transmitter 2 and obtaining the consent before starting the transmission with the optical transmitter 2. Good.

FIG. 11 shows an example of one-to-N communication in a loop network. Communication between the station (300) and the subscribers (301) and (302) is performed using loop optical fibers (315) and (316). In the normal state, communication is performed using only the optical fiber (315), and when the optical fiber (315) is disconnected, the optical fiber (31) is used.
Use 6). When both cores are broken, optical fiber
This is a network that uses both (315) and (316) to perform a loopback operation. Station of the optical transmitter 1 (303), (309) performs intensity modulation by the wavelength lambda _1. Optical transmitter 2 (304), (310)
Performs a frequency modulation or phase modulation lambda _n in the vicinity of the wavelength lambda _1. Usually, the optical transmitter 1 (303) and the optical transmitter 2 (304)
Operates, and is combined by the optical distributor (311), and the optical fiber
A signal is sent to (315). The subscribers (301) and (302) input and output optical signals using directional couplers (317), (318), (325) and (326). The coupling amount of these directional couplers is 10%
Use a material having a small transmission loss. Each of the optical receivers 1 (312) and (329) has a built-in wavelength filter capable of receiving light having wavelengths of λ ₁ and λ _n , and receives signals from the intra-office optical transmitter 1. The optical receivers 2 (322) and (330) are the intra-office optical transmitters 2
Is received. Also, the optical transmitter 1 (323),
The wavelength of (331) is λ ₂ and is a wavelength that is blocked by the wavelength filter built in the optical receivers 1 (321) and (329) of the own station. The optical transmitters 1 (323) and (331) are of the intensity modulation system and transmit to the optical receivers 1 (307) and (305) of the station. Further, the optical transmitter 2 (324), a vicinity of the wavelength lambda _m is lambda ₂ of (332),
A signal is transmitted by frequency modulation or phase modulation, and received by the optical receivers (308) and (306) in the station. (319), (320), (3
27) and (328) are optical distributors.

In this network, basic communication can be realized using TDMA in the transmission system of the optical transmitter 1 and the optical receiver 1. Large-capacity communication is performed by the transmission system of the optical transmitter 2 and the optical receiver 2. At this time, control is performed by the system of the optical transceiver 1, and a system in which the signals of the optical transmitter 2 of each subscriber do not collide is constructed. it can.

[0021]

According to the present invention, the conventional intensity modulation and
A new service can be added without affecting the network for direct detection and using the transmission path of the network. In addition, the network using the present invention is efficient because it is only necessary to improve the facilities of stations that require a large capacity.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of the present invention.

FIG. 2 illustrates the principle of the present invention.

FIG. 3 is a diagram showing a configuration of a receiver used in the present invention.

FIG. 4 is a diagram showing another configuration of the receiver used in the present invention.

FIG. 5 is a diagram showing a frequency distribution of the output of the amplifier.

FIG. 6 is a configuration diagram of another network to which the present invention can be applied;

FIG. 7 is a diagram showing another embodiment of the present invention.

FIG. 8 is a diagram showing another embodiment of the present invention.

FIG. 9 is a diagram showing a frequency distribution used in each transmission path.

FIG. 10 is a configuration diagram when the present invention is applied to another network.

FIG. 11 is a configuration diagram when the present invention is applied to another network.

FIG. 12 is a diagram for explaining a conventional optical communication system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical transmitter 1 2, 4, 5, 6 ... Optical fiber 3 ... Star coupler 7, 8, 9 ... Optical receiver 1 20 ... Optical receiver 2 21, 22 ... Optical distributor 25 ... Optical receiver 2

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ identification code FI H04B 10/20 H04J 14/04 14/06 H04L 12/44 (58) Investigated field (Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 H04L 12/44

Claims (5)

(57) [Claims] 1. A first transmitter that intensity-modulates and outputs a digital signal and a first receiver that directly detects the intensity-modulated digital signal, using a first wavelength to perform intensity modulation at a first wavelength.
A first transmission system that performs optical communication of digital signals by a direct detection method, and a phase modulation method or a frequency modulation method with a constant light intensity by a second wavelength using a second transmitter and a second receiver. Multiplexes with the second transmission system for optical communication ,
Optical communication system that transmits information using multiple signals
Thus, the first receiver receives the multiplexed signal as it is,
Output of the first transmitter from the light intensity modulated signal of the multiplex signal
Signal, the second receiver receives the multiplexed signal as it is,
The second receiver has an optical frequency selection compared to the first receiver.
The second transmitter by utilizing the high possibility
An optical communication system characterized by extracting the output signal of 2. The second receiver according to claim 1, further comprising: a local oscillation oscillator for oscillating light having a reference wavelength; and a multiplexed signal for transmitting information .
Combining light as it is and light from the local oscillation oscillator 2
A 2 × 2 combiner, and a plurality of photoelectric conversion elements for receiving the light combined by the 2 × 2 combiner and converting the light into an electric signal, and extracting a difference signal between the plurality of photoelectric conversion elements. The optical communication system according to claim 1. 3. The second receiver comprises a heterodyne receiver, wherein the second transmission system has an intermediate band between the second transmission system and a band equal to or more than a base band band of the first transmission system. 3. The optical communication system according to claim 1, wherein a frequency is set. 4. The first digital signal is intensity-modulated and output.
A first transmitter and the intensity modulated first digital
A first wavelength using a first receiver for directly detecting the signal;
Of the first digital signal by intensity modulation and direct detection
The first transmission system performing the optical fiber communication has the second transmission system.
A second transmitter for intensity-modulating and outputting a digital signal
Directly detecting the intensity-modulated second digital signal of
And a second wave different from said first wavelength using two receivers
The first digital signal by intensity modulation and direct detection
Transmission system for performing communication of signals using the optical fiber
Is an optical communication system that is multiplexed using an optical multiplexer / demultiplexer.
And further using a third transmitter and a third receiver to perform a third
Phase modulation or frequency modulation method with constant light intensity depending on wavelength
The third transmission system that performs the communication of the formula using the optical fiber is
By adding the output signal of the first transmitter,
The output signal of the second transmitter and the output signal of the third transmitter
A multiplexed signal obtained by multiplexing force signals passes through the optical fiber, and the first signal is converted from the multiplexed signal to the first signal by the optical multiplexer / demultiplexer .
The output signal of the transmitter is separated and the first receiver is connected to the output of the separated first transmitter.
Receiving the power signal, and the light intensity modulated signal of the multiplexed signal after the separation is the second signal.
The third transmission is applied to the light intensity modulated signal of the output signal from the transmitter.
Add the signal of constant light intensity, which is the output signal from the transmitter
Are those obtained by the second receiver, receiving remains multiplexed signals after the separation
Then, from the light intensity modulated signal of the separated multiplexed signal,
2 and the third receiver receives the output signal as the multiplexed signal after the separation.
And the third receiver has a higher optical frequency than the second receiver.
By utilizing the fact that the wave number selectivity is high, the third
Optical communication characterized by extracting an output signal of a transmitter
method. 5. The optical communication system according to claim 1, wherein a 1: N star coupler is arranged in the middle of said optical fiber .